Rep. John Mica, a Florida Republican who convened the Transportation subcommittee hearing, said he's concerned that growing numbers of drivers on U.S. roadways are increasingly impaired with a mix of drugs and alcohol. But with no test to determine if a driver is high on THC, the psychoactive ingredient in marijuana, it's nearly impossible to gauge the danger. Instead, he said, it's only after a fatal crash that investigators can determine if a driver has measurable levels of THC in his bloodstream.

"There is no standard test for drivers," Mica said. "We have no acceptable test and no way of telling if people are impaired."

Rep. Gerald Connolly, D-Va., countered that it's just as possible that marijuana doesn't pose as dangerous a risk as alcohol for drivers, suggesting that high drivers may slow down rather than recklessly speed.

Connolly said studies suggest the effects of pot can dissipate in relatively short order.

The concern about high drivers, train conductors or pilots centers on the growing availability of marijuana for both recreational users and those with a doctor's prescription. In Colorado and Washington state adults can buy marijuana from a regular pot shop, while residents in nearly two dozen other states still need a prescription.

Regardless of how drivers obtain marijuana, government officials told Congress that the drug is still off-limits for those pilots, conductors, commercial drivers and others whose jobs are in any way regulated by Washington.

The acting director for the Transportation Department's Office of Drug and Alcohol Policy and Compliance, Patrice M. Kelly, said millions of people in "safety-sensitive" jobs, such a pilot or subway conductor, have been reminded that they can't use pot regardless of the laws in their home state.